This invention relates to radio direction finders and, more particularly, to a radio direction finding method and system which enables an accurate determination of the true bearing of a radio target by cancelling errors caused by group delays through the receiving, detection and evaluation process.
Radio direction finders utilizing the principle of periodic scanning of a circular antenna array inherently display a bearing error related to the group delay through the receiving, detection and evaluation process. Thus, a finite time elapses between the instant when the sampled antenna (or antennas) cross the target line and the moment when the signal from that sample is compared with the fixed reference angle.
When a circular antenna array is scanned to determine the direction, with respect to a reference direction, of a radio wave arriving from a radio target, a signal from the target is received at the antenna when in a given position, but it does not become detected until after a time which corresponds to the group delay of the system. At the time when the detected signal is compared with the reference, the sampled antenna has advanced to a new position, and the indicated bearing angle differs from the true bearing angle by an increment .theta. corresponding to the angle swept by the antenna during the group delay interval T.sub.O.
In a conventional radio direction finder, based on periodic scanning of a circular antenna array, the group delay error is corrected by displacing the phase of the reference by a fixed constant corresponding to .theta..sub.o in such a way as to make the bearing indicator read the true bearing angle .theta. directly. This method has an advantage of simplicity but it is subject to errors in the assumption that .theta..sub.o can be maintained constant.
The factors which influence the value of .theta..sub.o =.omega.t.sub.o are:
(i) The angular velocity of scanning, .omega.; PA1 (ii) The group delay t.sub.o of the receiver subsystem.
Item (i) can be kept constant with sufficiently high accuracy.
Item (ii), however, largely depends on the passband filter characteristics of the receiver where the group delay will vary with the tuning frequency inside the passband. Depending on the design of the filter, a practical variation may be some .+-.30% of the centre value of t.sub.o. This means, that .theta..sub.o could similarly vary by .+-.30%, as the signal is detuned, and the fixed correction applied to the reference angle is no longer adequate.
Maintaining the receiver tuned exactly to the centre frequency is not always practical for the following reasons:
(i) The target transmitter may be drifting in time;
(ii) The apparent transmitter frequency may vary with the target's movement relative to the DF site due to the Doppler effect;
(iii) The intelligence modulation of the target transmission may exhibit an assymetrical spectrum (such as SSB or suppressed carrier, etc.) and the average group delay may differ from that at the centre frequency.